In recent years, with the development of micro- and minicomputers, the expense of computer peripherals has often outweighed the expense of the associated computer itself. This has been especially true of electronic drafting devices and, as a result, efforts have been made to reduce costs by reducing the number of electromechanical parts required in the drafting device and by relying on as many passive elements as possible. This philosophy has been particularly predominant in the development of pen-holding and pen-changing devices but has not seen concerted application in the area of pen-sensing devices.
In multi-pen plotters it is important for the plotter to sense the location of all of the pens in the device in order to avoid trying to select a pen which does not exist, to avoid writing without a pen, and to eliminate pen-to-pen collisions. Prior art solutions to the pen sensing problem have use mechanical switches or optical detectors. In the case of mechanical switches, each sensor requires a switch, adjustment and considerable and the switches themselves are prone to reliability problems. Optical sensors, too, require considerable assembly time and adjustment and the cost of photodetectors is not insignificant. Additionally, paper dust and other foreign particles are known to build up on the surfaces of the photodetectors thereby causing interference with their operation. Both of these prior art solutions to the sensing problem are known to be expensive to implement.